The invention relates to a method and an apparatus for making a parison for blow molding a hollow article made of thermoplastic material, in particular a bottle. In the invention which a parison having a closed bottom is used, in the end of the parison opposite the closed bottom is heated to welding and shaping temperature which end is inserted into a finish mold form including a forming mandrel. The forming mandrel is axially moved forwardly into the parison to urge the finish portion of the parison into contact with the finish mold. Then the final finish profile suitable for setting up a closure is shaped by applying a pressure medium.
In the prior art (German Pat. No. 25 43 640) of forming the finish portion of a bottle a tapered mandrel enters the free end of a cylindrical parison which end has been heated to welding and shaping temperature. The tapered mandrel displaces the heated plastic material partly into the profile of an outer mold, whereupon the final shaping of the closure thread profile is obtained by air pressure which forces the heated material into contact with the outer form. The known method particularly permits production of a finish of a diameter which is larger than the diameter of the original parison.